An electrical power network includes various power distributing elements together with certain loads. The properties of the power distributing elements are generally specified by the manufacturer for 60-Hz, using such standard measurements as open-circuit impedance and short-circuit admittance. These characteristic immitance tests cannot be duplicated without affecting the normal on-line operation of the network because the network loads must be disconnected before either open- or short-circuit tests can be performed. However, in certain power distribution or conversion networks, as well as other electrical networks and pieces of equipment, it would be advantageous to be able to perform standard immitance tests and measurements while the various elements of the network are in operation.